“Gravitation.
The force that causes objects to drop and water to run downhill is the same
force that holds the Earth, the Sun, and the stars together and keeps the Moon
and artificial satellites in their orbits. Gravitation, the attraction of all
matter for all other matter, is both the most familiar of the natural forces and
least understood.

Gravity is the weakest of the four forces
that are currently known to govern the way physical objects behave. The other
three forces are electromagnetism, which governs such familiar phenomena as
electricity and magnetism; the “strong force” which is responsible for the
events in nuclear reactors and hydrogen bombs; and the “weak force,” which
is involved with radioactivity. Because of its weakness, gravity is difficult to
study in the laboratory.

Despite its weakness, gravitation is
important because, unlike the three other forces, it is universally attractive
and also acts over an infinite distance. Electromagnetic forces are both
attractive and repulsive and as a result generally cancel out over long
distances. The strong and weak forces operate only over extremely small
distances inside the nuclei of atoms. Thus, over distances ranging from those
measurable on Earth to those in the farthest parts of the universe,
gravitational attraction is a significant force and, in many cases, the dominant
one.

Both Sir Isaac Newton in the 17th century
and Albert Einstein in the 20th century initiated revolutions in the study and
observation of the universe through new theories of gravity. The subject is
today at forefront of theoretical physics and astronomy.

Late in the 17th century Newton put forward
the fundamental hypothesis that gravity that makes objects fall to Earth and the
force that keeps the planets in their orbits are the same. In the early 1600s
the German astronomer Johannes Kepler described three laws: first, all planets
move in ellipses with the Sun at one focus; second, a line between the Sun and a
planet would sweep out equal areas of the ellipse during equal times; third, the
square of the period of any planet (the quantity of the time it takes to go
around the Sun multiplied by itself) is proportional to the cube of its average
distance to the Sun--that is, the average distance multiplied by itself twice.

In his book ‘Principia Mathematical’,
published in 1687, Newton showed that both Kepler’s laws and Galileo’s
observations of Earth’s gravity could be explained by a simple law of
universal gravitation. Every celestial body in the universe attracts every other
celestial body with a force described by F = G Mass 1 Mass 2 / Radius squared in
which F is the force, m1 and m2 are the masses of the two gravitating objects, R
is the distance between them, G is the gravitational constant (6.67 x 10^ -8
dyne cm sq./gm sq.).

Also in the ‘Principia Mathematica’
Newton mathematically defined the concept of “force” to be equal to the mass
of an object on which the force is applied, multiplied by the acceleration that
results from the force, or F = MA, in which A is acceleration. Because the
gravitational force increases proportionately to the mass of the object that is
accelerated, any object, no matter what its mass, accelerates equally if placed
at the same distance from another mass. Galileo observed that all objects on the
Earth are accelerated by the planet’s gravity to the same extent.

Newton
demonstrated mathematically that the law gravitation he proposes predicts that
the planets follow Kepler’s three laws. Newton vision of a world governs by
the simple, unalterable laws exerted a powerful influence for more than a
century.

If the planets are attracted to the Sun by
gravity, why do they not fall in? Newton showed that if the velocity is high
enough, a planet will always be accelerating toward the Sun without ever leaving
its orbit. This is because an object’s motion is the result of both its
previous direction of travel and speed-- that is, its velocity and acceleration
applied to it. Just as a rock whirling at the end of a string as long as it is
whirled fast enough, so objects in a gravitational field remain in their orbits
if they are moving fast enough.

Gravity
not only keeps planets and moons in their orbits, but holds them together. It
also played a dominant part in their creation. The Sun, for example, produces
the heat and light needed for life on Earth through nuclear reaction deep in its
interior. These same reactions would blow the Sun apart if it were not for the
immense force of its self-gravitation holding it together. Some 5 billion years
ago the Sun and planets contracted out of a diffused cloud of dust and gas,
again compressing themselves under the influence of their own increasing
gravitational fields. In the same way the huge galaxies and clusters of
galaxies, consisting of trillions of stars, are bound by gravity and were formed
primarily by gravitational contraction, through other forces-- such as pervasive
magnetic fields in space-- probably played a role as well.

Newton’s laws do not explain why all
objects attract all others. By 1916 the theoretical physicist Albert Einstein
had formulated a new theory of gravity that attempted to explain its actual
nature. In his theory, called general relativity, gravity does not exist as a
real force. Instead, each mass in the universe bends the very structure of space
and time around it, somewhat as a marble sitting on a very thin piece of rubber
does. This distortion of the space surrounding each object in turn bends the
path of all objects, even those possessing no mass at all such as photons.

Despite the success of Einstein’s theory,
much remains unknown about gravity. Still unanswered are questions about its
relation to the other three forces of nature, why it is so much weaker, and why
matter creates the curvature of space around it. These and other fundamental
questions about gravity continue to be the subject of theoretical work by
scientists.”